• International Journal of Automotive Technology
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• 제5권4호
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• pp.257-268
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• 2004

The wavelet analysis method is introduced in this paper to study the nonstationary vibration of vehicles. A new road model, a so-called time domain correlated four-wheel road roughness, which considers the coherence relationships between the four wheels of a vehicle, has been newly developed. Based on a vehicle model with eight degrees of freedom, the analysis of nonstationary random vibration responses was carried out in a time domain on a computer. Verification of the simulation results show that the proposed road model is more accurate than previous ones and that the simulated responses are credible enough when compared with some references. Furthermore, by taking wavelet analysis on simulated signals, some substantial rules of vehicle nonstationary vibration, such as the relationship between each vibration level, and how the vibration energy flows on a time-frequency map, beyond those from conventional spectral analysis, were revealed, and these will be of much benefit to vehicle design.

• Structural Engineering and Mechanics
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• 제20권5호
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• pp.527-542
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• 2005

The main objective of this study is to examine the accuracy of the complete quadratic combination (CQC) rule with the modal responses defined by the ordinates of the uniform hazard spectra (UHS) to evaluate the peak responses of the multi-degree-of-freedom (MDOF) systems subjected to nonstationary seismic excitations. For the probabilistic analysis of the peak responses, it is considered that the seismic excitations can be modeled using evolutionary power spectra density functions with uncertain model parameters. More specifically, a seismological model and the Kanai-Tajimi model with the boxcar or the exponential modulating functions were used to define the evolutionary power spectral density functions in this study. A set of UHS was obtained based on the probabilistic analysis of transient responses of single-degree-of-freedom systems subjected to the seismic excitations. The results of probabilistic analysis of the peak responses of MDOF systems were obtained, and compared with the peak responses calculated by using the CQC rule with the modal responses given by the UHS. The comparison seemed to indicate that the use of the CQC rule with the commonly employed correlation coefficient and the peak modal responses from the UHS could lead to significant under- or over-estimation when contributions from each of the modes are similarly significant.

• Structural Engineering and Mechanics
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• 제66권1호
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• pp.139-149
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• 2018

Many practical engineering problems are associated with nonlinear systems subjected to nonstationary random excitations. Equivalent linearization methods are commonly used to seek for approximate solutions to this kind of problems. Compared to various approaches developed in the frequency and mixed time-frequency domains, though directly solving the system equation of motion in the time domain would improve computation efficiency, only limited studies are available. Considering the fact that the orthogonal functions have been widely used to effectively improve the accuracy of the approximated responses and reduce the computational cost in various engineering applications, an orthogonal-function-based equivalent linearization method in the time domain has been proposed in the current paper for nonlinear systems subjected to nonstationary random excitations. In the numerical examples, the proposed approach is applied to a SDOF system with a set-up spring and a SDOF Duffing oscillator subjected to stationary and nonstationary excitations. In addition, its applicability to nonlinear MDOF systems is examined by a 3DOF Duffing system subjected to nonstationary excitation. Results show that the proposed method can accurately predict the nonlinear system response and the formulation of the proposed approach allows it to be capable of handling any general type of nonstationary random excitations, such as the seismic load.

• Structural Engineering and Mechanics
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• 제26권6호
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• pp.709-723
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• 2007

Considering the vast usage of time-history dynamic analyses to calculate structural responses and lack of sufficient and suitable earthquake records, generation of artificial accelerograms is very necessary. The main target of this paper is to present a novel method based on nonstationary Kanai-Tajimi model and wavelet transform to generate more artificial earthquake records, which are compatible with target spectrum. In this regard, the generalized nonstationary Kanai-Tajimi model to include the nonstationary evaluation of amplitude and dominant frequency of ground motion and properties of wavelet transform is used to generate ground acceleration time history. Application of the method for El Centro 1940 earthquake and two Iranian earthquakes (Tabas 1978 and Manjil 1990) is presented. It is shown that the model and identification algorithms are able to accurately capture the nonstationary features of these earthquake accelerograms. The statistical characteristics of the spectral response of the generated accelerograms are compared with those for the actual records to demonstrate the effectiveness of the method. Also, for comparison of the presented method with other methods, the response spectra of the synthetic accelerograms compared with the models of Fan and Ahmadi (1990) and Rofooei et al. (2001) and it is shown that the response spectra of the synthetic accelerograms with the method of this paper are close to those of actual earthquakes.

• 대한토목학회논문집
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• 제13권4호
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• pp.65-75
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• 1993

본 연구는 규모가 큰 지진하중에 대한 해양 가이드 타워의 비정상과정 거동해법에 주안점을 두었으며, 아울러 작은 파랑이나 조류하중이 동시에 작용할 수 있는 경우도 고려하였다. 지진에 의한 지반운동의 비정상특성은 정상과정 성분에 지수함수의 시간포락함수를 곱하는 형태로 모형화하였다. 동적거동의 비정상과정 분산값을 해석적인 방법으로 산출하였다. 운동방정식에 계류장치의 비선형 복원력과 파동에 의한 비선형 점성저항력을 추계론적 최적화기법으로 선형화하여 동적해석을 수행하였다. 지진의 발생기간동안 예상되는 중급의 파랑하중에 의한 영향을 최대거동 산정시 고려하였다. 중급 파고조건에 대한 파랑하중은 지속기간이 상당히 길어서 정상과정으로 취급할 수 있으므로 이에 대해서는 주파수 영역해석을 사용하여 동적거동을 산출하고 이 결과를 지진에 의한 비정상과정거동에 반영하였다. 예제해석으로부터 비정상과정 해석방법을 검증하고, 지진과 파랑 및 조류하중의 각 성분이 전체응답에 미치는 영향을 분석하였다.

• 전산구조공학
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• 제5권3호
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• pp.127-137
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• 1992

해양 가이드 타워를 대상으로 하여 지진하중에 대한 심해용 구조물의 비정상 거동해법에 대하여 연구하였다. 지반운동의 비정상 특성을 정상과정 성분에 시간포락함수가 곱해진 형태로 모형화하였으며, 구조물의 비정상 거동은 시간종속 분산함수로 구하였다. 지진의 시간포락함수와 정상과정 성분의 자기상관함수를 지수함수로 수식화하여 나타냄으로써, 구조물 거동의 시간종속 분산값을 해석적인 방법으로 구할 수 있는 기법을 개발하였다. 시간에 따른 거동의 분산값을 기본자료로 하여 지진의 발생기간 동안 예상되는 최대거동을 구하였으며, 이 값을 구조물 거동을 정상과정으로 가정하여 산정한 결과와 비교 분석하였다.

• Journal of Mechanical Science and Technology
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• 제18권9호
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• pp.1565-1571
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• 2004

Ride characteristics of a vehicle moving on a rough ground with changing travel velocity are analyzed in this paper. The solution is difficult due to the non-stationary characteristics of the problem. Hence a new technique has been proposed to overcome this difficulty. This new technique is employed in the analysis of ride characteristics of a vehicle with changing velocity in the time/frequency domain. It is found that the proposed technique gives successful results in modelling non-stationary responses in terms of a series of stationary responses.

• Smart Structures and Systems
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• 제23권1호
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• pp.61-69
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• 2019

Offshore structures are generally exposed to harsh environments such as strong tidal currents and wind loadings. Monitoring the structural soundness and integrity of offshore structures is crucial to prevent catastrophic collapses and to prolong their lifetime; however, it is intrinsically challenging because of the difficulties in accessing the critical structural members that are located under water for installing and repairing sensors and data acquisition systems. Virtual sensing technologies have the potential to alleviate such difficulties by estimating the unmeasured structural responses at the desired locations using other measured responses. Despite the usefulness of virtual sensing, its performance and applicability to the structural health monitoring of offshore structures have not been fully studied to date. This study investigates the use of virtual sensing of offshore structures. A Kalman filter based virtual sensing algorithm is developed to estimate responses at the location of interest. Further, this algorithm performs a multi-sensor data fusion to improve the estimation accuracy under non-stationary tidal loading. Numerical analysis and laboratory experiments are conducted to verify the performance of the virtual sensing strategy using a bottom-fixed offshore structural model. Numerical and experimental results show that the unmeasured responses can be reasonably recovered from the measured responses.

• 한국지진공학회:학술대회논문집
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• 한국지진공학회 1998년도 춘계 학술발표회 논문집 Proceedings of EESK Conference-Spring 1998
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• pp.123-130
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• 1998

The probability of unseating failure of the bridge spans under earthquakes is investigated. Seismic excitations are simulated as nonstationary processes by combining a stationary process and an intensity function. For computational convenience, a simplified single-degree-of-freedom model is adopted, which retains the dynamic characteristics of the original brige motion in concern. The time history analysis for the developed single degree-of-freedom model are carried out to evaluate the response processes, and the probabilistic characteristics of response displacements are evaluated. The reliability analysis of the bridge against the unseating failure is performed with the statistical information of the maximum displacements of responses.

• Nuclear Engineering and Technology
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• 제55권3호
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• pp.968-980
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• 2023

The dynamic structural responses are sensitive to the time-frequency content of seismic waves, and seismic input motions in time-history analysis are usually required to be compatible with design response spectra according to nuclear codes. In order to generate spectra-compatible input motions while maintaining the intrinsic non-stationarity of seismic waves, an improved time-domain approach is proposed in this paper. To maintain the nonstationary characteristics of the given seismic waves, a new time-frequency envelope function is constructed using the Hilbert amplitude spectrum. Based on the intrinsic mode functions (IMFs) obtained from given seismic waves through variational mode decomposition, a new corrective time history is constructed to locally modify the given seismic waves. The proposed corrective time history and time-frequency envelope function are unique for each earthquake records as they are extracted from the given seismic waves. In addition, a dimension reduction iterative technique is presented herein to simultaneously superimpose corrective time histories of all the damping ratios at a specific frequency in the time domain according to optimal weights, which are found by the genetic algorithm (GA). Examples are presented to show the capability of the proposed approach in generating spectra-compatible time histories, especially in maintaining the nonstationary characteristics of seismic records. And numerical results reveal that the modified time histories generated by the proposed method can obtain similar dynamic behaviors of AP1000 nuclear power plant with the natural seismic records. Thus, the proposed method can be efficiently used in the design practices.